218 



CASSELL'S POPULAR GARDENING. 



and various mineral matters already alluded to under 

 the head of root-action, such as potassium, calcium, 

 iron, &c., which are also in their degree indispens- 

 able, and others that are less so. In considering 

 the manner in which these substances are introduced 

 into the plant, we shall gain an idea of the in- 

 terdependence of roots and leaves, and in tracing 

 their combinations in the plant we shall of necessity 

 learn something of the nature of leaf-action, and 

 especially of the influence of the green matter called 

 chlorophyll. In the first place it must be repeated 

 that the elements above referred to are some of them 

 introduced by the agency of the roots and root-hairs ; 

 others by the mediation of the leaves. In neither 

 case are the elements received in their uncombined 

 state, but in combination with other elements. 



These matters are often spoken of loosely as con- 

 stituting the food of plants. Plants are said to 

 feed on carbon, oxygen, hydrogen, and nitrogen, 

 together with the mineral matters above men- 

 tioned. Strictly speaking this is an error. These 

 elements do not, as taken into the plant, suffice 

 to nourish it any more than the oxygen, nitrogen, 

 carbon, &c., of which beef and bread are made 

 up, directly nourish us. In our case even the beef 

 and the bread are not of themselves foods. It is not 

 till they have undergone in the stomach and intes- 

 tines certain complicated chemical changes that they 

 become actually fit to serve as food. They furnish 

 the material out of which food is made, but the}' are 

 not the food itself. So with the plant, it is not till 

 the materials taken in by the root and by the leaf 

 conjointly have undergone chemical changes carried 

 on in, or at least in connection with, the green 

 matter, when exposed to light, that they become 

 fit to nourish the plant, repair its waste, and form 

 new tissue. The leaf, then, or the green matter in 

 it, under the conditions we have mentioned, is the 

 great agent in converting the materials of food into 

 food itself. Let us, then, have a little more respect 

 for the leaves, tend them, keep them clean and 

 unobstructed, especially their under surface, and 

 afford them every facility for fulfilling their office as 

 purveyors, cooks, builders, and purifiers of the plant. 



The Use of Chlorophyll. — It will be noticed 

 that, in speaking of the leaves as the concocters 

 and digesters of food, we have laid special stress 

 on the green matter, and for the reason that 

 in some way or another, exactly how is still a 

 matter of controversy, this chlorophyll plays an 

 essential part. But chlorophyll is not a uni- 

 versal constituent in plants. It is not found in 

 every cell even of green plants. It is entirely 

 wanting in all the great host of fungi. How, then, 

 do cells destitute of chlorophyll live ? How do 



f imgi feed in the absence of this green matter ? Ib 

 answer to these questions, it may be said that the 

 colourless cells of green plants live at the expense 

 of their more richly endowed neighbours. They 

 are, as it were, parasitic upon them. Thus, in the 

 case of fungi we know but too well that they do not 

 collect and prepare their own food, but that they 

 either live a parasitic existence on other plants, 

 or derive their noiuislmient from the decay of 

 animal or vegetable substances. The green plants 

 absoib the raw material, and, under the agency of 

 light, transform it into food. The fungi feed upon 

 the matter already manufactured by others, and are 

 thus comparatively independent of light. Cells 

 destitute of chlorophyll can feed and grow in the 

 dark, but they cannot collect raw material nor manu- 

 factiu"e it into food for themselves. Cells provided 

 with chlorophyll can grow and digest in the dark, 

 but they cannot convert the inorganic matter derived 

 from soil or atmosphere into food unless exposed 

 to light. 



Reverting to the raw materials of plant-food, it 

 may be repeated that the carbon is derived by the 

 leaves from the atmosphere in the form of carbonic 

 acid, the hydrogen and oxygen as water are ab- 

 sorbed by the roots, and in a gaseous form, in com- 

 bination, by the leaves. Nitrogen gains access in the 

 form of nitrates or as ammonia salts, also by the 

 roots, by which likewise the various mineral in- 

 gredients are taken up as previously explained. 

 Oxygen is thus taken up both by the leaves and by 

 the roots : in the former case from the atmosphere, 

 as a gas essential to respiration ; in the latter case 

 in combination with other matters, and destined to 

 take part in the transfonnation of the food-material 

 into food. 



Influence of Light and Heat. — The first 

 and most striking effect observed when chloro- 

 phyll is exposed to light is, as has been above said, 

 the emission of oxygen gas. This is supposed to 

 result from the splitting up or disintegration of the 

 carbonic acid gas of the atmosphere into oxygen, 

 which is given off, and carbon, which is retained, the 

 proportion between the carbonic acid absorbed and 

 the oxygen given off being in this case definite. As 

 the solid matter of plants consists in by far larger 

 proportion of carbon than of any other ingredient, 

 the desirability of giving leaves free access to light 

 becomes obvious, and in the laboratory the chloro- 

 phyll is found to do its work most thoroughly when 

 subjected to the yellow or light-giving parts of the 

 spectrum, and least so when exposed to the chemical 

 rays, such as violet or blue. The amount of exj^o- 

 sure required varies according to structure and other 

 circumstances ; shade-lo^-ing plants, like the Ferns, 



